The present invention relates to systems for transmission, by means of digital signals, of data concerning the actuation of a member used for operation of a vehicle. It relates in particular to the problem of fidelity of transmission of such data in on-board systems operating in real time.
This problem occurs, for example, in the field of control of vehicles such as an automobile or airplane. In such vehicles, the actuation of a certain number of electromechanical members is “assisted”, in the sense that the vehicle operator controls the operation of these members by sending an electrical signal to a servo unit responsible for actuating these members by means of assistance mechanisms such as hydraulic systems or electromagnets, in conformity with the said electrical control signal.
The first electrical controls of these type were formed by analog signals, but digital signals are being increasingly used to support control orders. In this case, the actions exerted by the vehicle operator are converted to the form of digital controls, which are then inserted in messages by an insertion unit; then these messages pass through one or more interconnection units until they reach one or more extraction units, in which the said digital controls are extracted from the said message; finally, the control orders are applied to one or more servo units (if necessary after they have been converted to analog signals for the servo units that need them).
These controls often concern vehicle members that must operate properly to ensure passenger safety. It is therefore very important to be sure that the messages are transmitted reliably between the insertion unit and each extraction unit. However, that poses a problem to the extent that various factors may affect the reliability of such transmission. These factors comprise in particular electronic noise (which may result from external electromagnetic perturbations or from fluctuations intrinsic to the electrical components used to transport the digital signals) and physical failure of one of these electrical components.
The problem considered hereinabove also occurs as regards the digital data representative of a physical measurement made by a sensor, when such measurement is taken into account by a servo unit in order to actuate a vehicle member. The same is true when a servo unit responsible for actuating a vehicle member must take into account digital information sent by a database or a processor.
Since transmission errors are inevitable in practice, the question that occurs is that of knowing how they can be taken into account in an on-board communication system operating in real time in such a way as to minimize the impact that such transmission errors may have on proper operation of the vehicle.
A known solution to this problem (for example, see the book edited by D. Powell entitled “A Generic Fault-Tolerant Architecture for Real-Time Dependable Systems”, Kluwer Academic Publishers, ISBN 0-7923-7295-6, Boston 2001, pages 46 to 50; or ISO Standard 11898-1, known under the name of “Controller Area Network”) comprises applying channel coding to the messages before they are transmitted and recovering these messages by decoding the received code words.
It is recalled that “channel coding” (the discussion here will be limited to “block” coding) comprises transmitting, to a receiver, “code words” formed by introducing a certain redundancy in the data to be transmitted. More precisely, by means of a code word there is transmitted the Information initially contained in a predetermined number k of symbols, known as “information symbols”, sampled in an “alphabet” of finite size q; from these k information symbols there is calculated a number n>k of symbols belonging to that alphabet and constituting the components of code words v=(v1, v2, . . . , vn). The set of code words obtained when each information symbol takes any value whatsoever in the alphabet comprises a kind of dictionary referred to as “code” of “dimension” k and “length” n.
In particular, when the size q of the alphabet is taken equal to a power of a prime number, this alphabet can be given a finite body structure known as “Galois body” (“Galois field” in English), denoted GF(q). For example, for q=2, GF(q) is a binary alphabet, and for q=28=256, GF(q) is an alphabet of octets (“bytes” in English).
A transmission error exists when a received word r differs from the corresponding code word v sent by the transmitter. It is said that transmission errors are caused by the “channel noise”.
After reception of the word r, the receiver first attempts to detect the possible presence of a transmission error. The codes that make it possible to detect that errors have occurred are known as “error detector codes”. It is these codes in which the present invention is primarily interested, although it would also be possible to envision correcting the transmission errors by resorting to “error corrector” codes. It will also be noted that certain codes also make it entirely possible both to detect the presence of errors and to correct them.
Regardless of the type of code used, it must be seen that the capacity of the code to detect or correct transmission errors is always limited. In general, the capacity for detection or correction of a code increases, for fixed k, with the “redundancy” (n−k). In particular, the probability of non-detection corresponds to the probability that a received word r, even though erroneous, is accidentally equal to a code word other than the transmitted word that is at the origin of this received word r; from this it is deduced that this probability is on the order of q−(n−k). Consequently, a predetermined error detection reliability in a digital communication system is traditionally achieved by adjusting the parameters q, n and k of an error detection code to conform to this order of magnitude.
However, the foregoing estimate of the probability of non-detection starts from the hypothesis that random errors are involved. As it happens, this hypothesis is invalidated in certain specific applications.
For example, let us return to the problem of making digital controls safe in a vehicle: let us therefore suppose, as explained hereinabove, that these controls are inserted in messages composed of Information symbols belonging to a certain alphabet, and let us suppose that these messages are provided with a certain channel coding. It will then be possible to consider that certain errors, such as those caused by electromagnetic fluctuations, are effectively of random nature and of short duration: in this case, it will be possible to consider reliably that the frequency at which such errors are not detected (in the coded message, and consequently in the decoded message) is equal to the product of the mean frequency of these fluctuations by the probability q−(n−k) of non-detection mentioned hereinabove.
On the other hand, let us suppose, again for the example of digital controls of a vehicle, that an electric component along the “transmission channel” definitively develops a fault, and that this fault unfortunately, additionally causes a non-detected error in a message. The rate of occurrence of such an event is always equal to the product of the rate of failure of such an electrical component by the probability q−(n−k) of non-detection. However, it is seen that, in this situation, if the operator then sends a second order identical to the first (which may be the case when the operator attempts to activate a certain electromechanical member in conformity with proper operation of the vehicle), then the new order will be affected by the same error as the preceding, and this new error again will be non-detected. The consequences that such a situation could provoke can be easily imagined.
A first known means for reducing the risk that such an event will occur is to double the electrical circuits in such a way that two copies of each message can be transmitted along two separate channels respectively. Under these conditions, it is known that an error is produced if, after decoding of the corresponding words along these two channels, it is found that the resulting two messages are not identical. This technical solution obviously has the disadvantage of doubling the installation costs of these electrical circuits. In addition it involves an increase not only in the space occupied by these circuits but also in the weight thereof, which may be very inconvenient in certain fields such as aviation.